The present invention relates to methods for encasing radioactive wastes in a sheath, the wastes being solidified with binders and stored in waste barrels and the sheath being resistant to leaching, water and aqueous solutions of natural salt minerals.
The leaching resistance of radioactive wastes to water and aqueous solutions of natural mineral salts has an important influence on the evaluation of long-term safety in the permanent storage of radioactive wastes in geological permanent storage facilities or in other types of storage systems. In the past, the solidification of radioactive wastes with respect to water and aqueous solutions was accomplished by mixing the wastes with bitumen or hydraulic binders. These radioactively contaminated masses subsequently were hardened in sheet-metal barrels. At some locations in the Federal Republic of Germany (Wurgassen, Stade), borate-containing liquid wastes from nuclear power plants are directly evaporated into barrels where they harden, upon cooling, into solid salt blocks which are water soluble. Solid contaminated wastes from laboratories, state collection stations and nuclear power plants are filled into barrels without further treatment and are then put into permanent storage. Such wastes are sometimes compacted by pressing.
In another technique, so-called lost concrete shields are used to hold a 200-liter waste barrel and limit the radiation energy at the surface of the containers to the permissible radiation values. However, these containers may be wetted by water and aqueous salt solutions. Moreover, the concrete has pores into which water and dissolved salts can penetrate. This may cause the concrete to be destroyed and the radioactivity to be leached out.
In all these cases, the radioactive wastes would be subjected directly to the influence of water or aqueous solutions if what is considered the worst possible accident, i.e., the hypothetical use of the penetration of water, were to happen at the permanent storage facility. Such an occurrence may result in partial leaching of the radioactivity contained in the barrels. Thus, it is possible that the radioactivity might spread through the water-filled mine facility and could possibly find its way into a layer leading to underground water supplies.
In addition to leaching, dangerous chemical attacks may occur mainly in the wastes that are bound with hydraulic binders. The clinker mineral, tricalcium aluminate (present in cement), will react with the sulfate ions of the aforementioned aqueous solutions to form tricalcium aluminate sulfate (ettringite). Due to the crystallization pressure which is present the structure of the binder would be destroyed and the result may be a crumbly mass. In view of the great surface enlargement of the radioactive waste, this also would increase the proportion of leached radioactivity. For evaporated solutions of borate-containing wastes or of compacted or uncompacted solid wastes, the entire radioactive content may be dissolved or washed out.